Environmental interests and an increasing demand for diesel fuel drive fuel producers to employ more intensively renewable sources in the manufacture of diesel fuel. In the manufacture of diesel fuels based on biological raw materials, e.g., renewable and bio diesel, the main interest has thus far concerned vegetable oils and animal fats comprising triglycerides of fatty acids. Long, straight and mainly saturated hydrocarbons of fatty acids correspond chemically to the hydrocarbons present in diesel fuels.
Plant oils and fats, as well as animal oils and fats can contain 0-30 wt-% of free fatty acids (FFAs). FFAs are formed during enzymatic hydrolysis of triglycerides especially when oil seeds are kept in humid atmosphere. Free fatty acids can also be formed during purification of bio oils and fats, especially during caustic wash, e.g., alkali catalyzed hydrolysis.
The fatty acid composition, size and saturation degree may vary considerably in feed stocks from different origins. The melting point of bio oil or fat is mainly a consequence of the saturation degree. Fats are more saturated than liquid oils and require less hydrogen for hydrogenation of double bonds.
Free fatty acids are very corrosive because of their acidity. They attack against materials they are in contact with which may cause corrosion on equipment used in processes involving FFAs, such as piping and reactors.
FFAs also promote undesirable side reactions. Unsaturated feeds and free fatty acids in bio oils and fats promote the formation of heavy molecular weight compounds, which may cause plugging of the preheating section and decrease catalyst activity and life. Double bonds in unsaturated FFAs also promote various other side reactions, such as oligomerisation/polymerization, cyclisation/aromatisation and cracking reactions. These reactions cause many problems in the process and deactivate catalysts, increase hydrogen consumption and reduce diesel yield.
Fatty acids may also promote the formation of heavy compounds. The boiling range of these heavy compounds is different from the range of diesel fuel and may shorten the life of isomerisation catalyst. Due to the free fatty acids contained in bio oils and fats, the formation of heavy molecular weight compounds are significantly increased compared to triglyceridic bio feeds which have only low amount of free fatty acids (<1%).
EP 1741768 A1 describes a process for the manufacture of diesel range hydrocarbons wherein a feed is hydrotreated in a hydrotreating step and isomerised in an isomerization step and a feed comprising fresh feed containing more than 5 w-% of free fatty acids and at least one diluting agent is hydrotreated at a reaction temperature of 200-400° C., in a hydrotreating reactor in the presence of catalyst, and the volume ratio of the diluting agent to fresh feed is 5-30:1. Hydrodeoxygenation, HDO, step is disclosed as a hydrogenolysis process for removing oxygen from oxygen containing compounds and FFA-feeds.
Free fatty acids are corrosive and they can attack against materials of unit or catalyst and promote some side reactions. Free fatty acids react very efficiently with metal impurities producing metal carboxylates, which promote undesirable side reaction chemistry.
The corrosiveness of FFAs present in renewable sources of hydrocarbons has limited use of renewable sources in manufacturing diesel fuel. The high concentration of FFAs in fresh feeds prevents feeding high concentrations of fresh feed into recycle feed without exposing for example feed system, heat exchanger and reactor to corrosion. The risk of corrosion is, for example, high above the reaction bed where fresh feed is mixed with the recycle, such as at HDO.
Corrosion rate is affected by factors such as materials used in the process, operating temperature, organic acid concentration and sulphide formation.
Undesired oxygen may be removed from fatty acids or their esters by deoxygenation reactions. The deoxygenation of bio oils and fats, which are oils and fats based on biological material, to produce hydrocarbons suitable as diesel fuel products, may be carried out by catalytic hydrotreating.
During hydrotreating, for example hydrodeoxygenation, oxygen containing groups are reacted with hydrogen and removed as water formed in the reaction. This reaction can require rather high amounts of hydrogen. Due to the highly exothermic nature of these reactions, the control of reaction heat can be important. Impure plant oil/fat or animal fat/oil, high reaction temperatures, insufficient control of reaction temperature or low hydrogen availability in the feed stream may cause unwanted side reactions, such as cracking, polymerisation, ketonisation, cyclisation and aromatisation, and coking of the catalyst. These side reactions also decrease the yield and the properties of the obtained diesel fraction.
EP 1741768 A1 also describes an improved process for the manufacture of diesel range hydrocarbons from bio oils and fats, wherein the hydrotreatment of triglyceride feedstock containing free fatty acids is carried out using dilution of fresh feed and reduced reaction temperature. The dilution agent is selected from hydrocarbons and recycled product of the process, i.e., product recycle or mixtures thereof. A ratio of at least 5:1 (recycle:fresh) significantly decreased the formation of high molecular weight products, when the feedstock containing 10 wt-% of free fatty acids (calculated from fresh oil) was used. Thus using at least 5:1 or 4:1 recycle ratio and reduced reaction temperature, free fatty acids can be processed without the need for deacidification and high quality hydrocarbons suitable for the diesel fuel pool were obtained.
When a feedstock with high levels of FFAs or other organic acids is used, it can be desirable to protect the system from corrosion. For example, the feed system, heat exchanger, and/or reactors are affected most as they are the first to be in contact with the feed with the highest FFA content. Controlling corrosion by corrosion-resistant materials is possible but results into high costs and extensive modifications of the units of the refinery.
Corrosion prevention includes previously been achieved by cladding the exposed surfaces, adding a corrosion inhibitor, anti-corrosion agents, or changing the materials (corrosion resistant material) used in the exposed surfaces. These methods are disclosed for example from US 2012/0053377.
US 2012/0216450 describes a method of converting feeds from renewable sources in co-processing with a petroleum feed using a catalyst based on nickel and molybdenum.
U.S. Pat. No. 8,440,875 describes a method of making a diesel fuel from a renewable feedstock. Ammonia or an amine compound is used to neutralize the organic acids in the renewable feedstock. The ammonia or amine compound should be removed from the product mixture before the isomerization zone so that it does not affect the isomerization catalyst. The use of ammonia or amine compounds could allow for the metallurgy reductions in the process equipment.
US 2012/0255223 discloses a process for conversion of FFA containing oils to biodiesel by converting high FFA containing feed stocks (FFA 20-85%) without pretreatment or purification to biodiesel in the presence of macro reticular and gel type acidic heterogeneous resin followed by transesterification in presence of homogeneous basic catalyst and separation of biodiesel and glycerin.
FR 2989006 A1 discloses a catalytic reactor with a quench device provided with tangential injection of a quench fluid.
Despite some attempts in the field to allow mixing corrosive fresh fluid feeds into recycled fluid feeds, there still remains a need for new methods and apparatuses that allow efficient mixing of the fresh and recycled feeds without need for extensive protection or diluting or neutralising agents. For example, high FFA feeds in manufacturing diesel range hydrocarbons from renewable source have previously been difficult or impossible to feed into recycled process stream in a cost-effective way without extensive use of additional neutralising or diluting agents, conversion, or protection of reactor parts by protective layers.